1. Field of the Invention
This invention relates to the recovery of a metal from a solution containing ions of the metal and a complexing agent for the ions by means of a metal-metal exchange reaction that yields a precipitate of the recoverable metal, which can then be separated from the solution. In one embodiment the invention relates to the recovery of silver from solutions that have been used to process silver halide photographic materials, e.g., spent fixing or bleach-fix solutions. More particularly, this invention is concerned with maximizing the yield of metal-metal exchange reactions useful for metal recovery.
2. Description Relative to the Prior Art
The major known methods for recovering metals from solutions containing ions of the metals are precipitation by chemical agents, electrolysis, and metal-metal exchange. The last-mentioned method comprises reducing a dissolved complex of ions of the metal to be recovered by contact with a more reducing metal, i.e., a metal which is more electropositive than the metal to be recovered. (A metal which is more reducing or more electropositive than another metal is a metal which is more easily oxidized than the other metal and, thus, is an effective reducing agent for cations of the other metal). The precipitate of the recoverable metal is then separated from the solution by known methods, such as filtration.
Many patents describe methods and apparatus for recovering metals by metal-metal exchange. For example, U.S. Pat. Nos. 1,194,056; 3,705,716; and 4,065,313 disclose methods and apparatus for recovering silver from used photographic processing solutions by an exchange reaction with a metal more electropositive than silver, such as iron or copper. These patents are hereby incorporated herein by reference.
One problem associated with the metal-metal exchange recovery method is that the yield obtained can be adversely affected by the presence in the solution of chemical species that would tend to reoxidize the recoverable metal already reduced by the metal-metal exchange reaction. In other words, when the purpose is to recover a metal from an ionic solution by a metal-metal exchange reaction, it is desirable that the reduced recoverable metal not be reoxidized by reaction with other chemical species. Maximum yields will not be obtained if such reoxidation reactions are allowed to proceed to a significant extent.
For example, as described in the above-identified patents, the exchange reaction with elemental iron to precipitate silver from used photographic processing solutions may be expressed as: EQU Fe.degree.+2Ag.sup.+ .fwdarw.Fe.sup.2+ +2Ag.degree.
If the ferrous ions formed as a result of this exchange reaction are oxidized by air to form ferric ions in the used processing solution, these ferric ions are capable of reoxidizing the reduced silver back to silver ions; and the tendency to do this is even more pronounced when the processing solution contains complexing agents for silver ions (such as the thiosulfate compounds typically found in photographic fixing baths). The same problem can arise when the processing solution from which silver is to be recovered is a bleach-fix solution that already contains ferric ions from the ferric ethylenediaminetetraacetic acid complexing agents frequently found in such solutions.
Some prior art processes attempt to avoid this problem by treating used processing solutions by a continuous (rather than batch) method wherein the solution flows path the reducing metal (e.g. steel wool) with silver reduction occurring at the reducing metal and the continuous flow of solution carrying away substances such as ferric ions which could otherwise reoxidize the reduced silver. For example, the methods disclosed in U.S. Pat. Nos. 2,194,056; 3,705,716; and 4,065,313 are of this type. These prior art methods, however, have their own disadvantages. For example, it is difficult to determine the precise moment at which the reducing metal is about to become exhausted and should be replaced with a new supply of reducing metal. The method described in U.S. Pat. No. 2,194,056 does not even attempt to do this; rather, according to that method the recovery cartridge is weighed and replaced when only 60 to 75 percent of the reducing metal has been exhausted, thus necessitating frequent cartridge replacement and considerable waste of reducing metal.
U.S. Pat. Nos. 3,705,716 and 4,065,313 determine when the reducing iron is exhausted by measuring either the silver ion concentration in the effluent from the cartridge (U.S. Pat. No. 3,705,716) or the change in the ratio of ferric to ferrous ions at the entrance and exit of the cartridge (U.S. Pat. No. 4,065,313). Unfortunately, by the time either of these methods has determined that the iron is exhausted, some processing solution containing significant concentration of silver ions has already left the cartridge, and the processing solution remaining in the cartridge at this point not only contains significant silver ion concentration, but may also have begun to reoxidize silver previously reduced by the metal-metal exchange reaction. Thus, these prior art methods can produce yields or recovered silver that are significantly less than the maximum possible.